Replies to This Discussion

Permalink Reply by Jim and Amelia Isaacs on February 10, 2025 at 3:01am

Hi Oliver,

I agree with your suspicions. Are all your engine compartment fuel lines covered in fire sleeve already? If so that should be sufficient protection from heat. Although some do, Rotax does not advise wrapping the exhaust for several reasons. And although some do, mostly for cost or convenience, Rotax does not advise routing the return line to anywhere other than a vented fuel tank. This is best way to help cool the fuel, and more importantly, release any trapped air, which will rise to the top then out of the vented tank. Trapped air has nowhere to escape to the atmosphere when it arrives in a gascolator. It will rise to the top of the gascolator- and be pulled back into the line to the carbs by the fuel pump. A related question, is your electric boost pump located directly under the fuel tanks and pushing fuel forward to the engine-driven pump? 

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Permalink Reply by Peter Eiberger on February 10, 2025 at 1:33pm

Are you running auto fuel or avgas? Auto fuel is far more prone to vapor lock than avgas and winter blend auto fuel is even more prone than summer blend.

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Permalink Reply by Oliver Reik on February 15, 2025 at 12:04pm

Hello Jim, hello Peter,

It turned out that that having the fuel restrictor jet of the fuel return line installed right on top of the gascolator was apparently the problem. We bought the hoses already crimped as a kit, there was therefore no easy way to change this.

As a 'quick fix' we installed a few feet of hose between the restrictor jet and the gascolator, to give the returned fuel some time to dissolve any bubbles, caused by cavitation, due to the rapid pressure drop at the restrictor jet.

Once we made this change, we were not able to reproduce the problem anymore, no matter how hard we tried.

 

Having looked deeper into this topic, I however also realized that it is not acceptable to return the fuel into the gascolator anyway, even though this appears quite common.

As per Rotax is it mandatory that the fuel is returned all the way back into one of the wing tanks. The fwf fuel lines also need to be insulated, even if hoses with an integral fire sleeve are being used.

We already insulated the fuel lines and will now also route the return line into one of the tanks.

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Permalink Reply by Jim and Amelia Isaacs on February 16, 2025 at 2:52pm

Thanks for the follow up Oliver, so to alert any future reader, who sells that kit? It may help prevent someone else from unknowingly installing a hazardous design. 

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Permalink Reply by Michel Ruf on March 6, 2025 at 3:40am

Hi Oliver,

I have the same setup with a 912 ULS like you with the return line into the gascolator. My gascolator sits behind PIC seat, might lead to better cooling. Restrictor at the fuel distribution block. Fuel pump after gascolator for compression of fuel. I have now a 100 hrs and not experiencing any problems with vapor lock.

I also didn't like the return line into the gascolator, but wings were already closed. So I chose this solution. How do you get back to the return port of the wing tanks now? Cutting new inspection openings and pass through rear spar?

I am also thinking about installing a header tank cause the wing tanks empty sometimes unevenly. In this case I need the return line anyway. But I do not like to cut openings in the perfect painted wing underside. Trying to figure out a easy solution, maybe to go with the return line trough the tank cap (Savanna Design). Still thinking...

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Permalink Reply by Oliver Reik on March 6, 2025 at 8:50pm

Hi Michael,

I spoke yesterday with the Rotax service center, who confirmed that our issues sounded vapor related, even though the ambient temperatures weren't all that high.

He however also said that factors like not having the orifice at the distribution block, but directly at the gascolator, which also happens to sit right next to the exhaust, might contribute to the issue.

That the problem became harder to reproduce, after we added additional insulation to the fuel lines and completely disappeared once we added a few feet of fuel line after the orifice in the return line might be indicative of that.

He however also stated that our setup might worsen vapor issues, as we are now trying to pump vapor around, what obviously doesn't work. Once the pump loses it 'grip' on the gas, the pressure collapses, vapor is even more likely to form and no fresh fuel will be pushed towards the carbs anymore.

Anyway, he also stated that if they would get an aircraft like ours in, the first thing would be to make it compliant with the Rotax installation manual, what means a return line to a header tank or, even better, to one of the wing tanks.

We want to go the wing tank route but are not quite sure yet whether this will be possible. The biggest concern is whether we will be able to cut a hole into the wing rib, adjacent to the return fitting in the tank, through the inspection opening.

We would also install a small panel in the wing's trailing edge, to route the return line.

A small header tank on the firewall, after the fuel selector, might be an option a well.

A header tank in the back would't really work for us as our CG is already very far aft. We would also not want to give up on the ability to select between the left and the right tank.

In retrospect, we should have installed a return line, as required by Rotax, right away.

What's the point in paying a premium for their R&D, only to deviate from their installation requirements?

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Permalink Reply by Michel Ruf on March 12, 2025 at 8:02am

Hi Oliver,

as my gascolator sits behind the pilot's seat and is sticking out of the fuselage, it might be better cooled than yours. I tried to find out if vapour is dissolved if it's cooled down in the gascolator, but found no answer. For sure I think it cannot return to the wing tanks through the supply hose. 

I totally agree with you that it was better to install a return line from begin. But at the time of building the wings it was not clear to me. Funny fact is that Rotax advises to install a return line since some years. All older installation do not have one.

I like the idea of an header tank. But it must also be vented and the vent might to be too short if the header is at the firewall. Guess the vent needs to be higher than the tanks.

I am still waiting for the day I have the nerves to cut an opening in one wing (I also would like to be on the save side despite I did not have any issues till now). As I remember there was a precut hole in the rib for the return port, but not totally sure. Plans might have an answer. 

  

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